Auto merge of #109849 - scottmcm:more-fieldidx-rebase, r=oli-obk

Use `FieldIdx` in various things related to aggregates

Shrank `AggregateKind` by 8 bytes on x64, since the active field of a union is tracked as an `Option<FieldIdx>` instead of `Option<usize>`.

Part 3/? of rust-lang/compiler-team#606

[`IndexSlice`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_index/vec/struct.IndexVec.html#deref-methods-IndexSlice%3CI,+T%3E) was added in rust-lang/rust#109787

Author: bors